Existing telecommunication transmission formats have typically been developed in the past when the operation of user's telecommunication equipment was always synchronized to the operation of the network that was used to transport communications between telecommunication equipment: the network provided the clock for its own operation and for the operation of the user's equipment. As a consequence, the formats were predicated on the assumption that data and control information is provided by the user's equipment to the network at a predetermined steady and unvarying--the nominal--rate.
Today, network-synchronized operation of user equipment is no longer universally practiced. More and more network users are demanding network-independent timing capability: the ability to provide their own timing for their equipment and, consequently, the ability for the network to accept data and control information for transport at a rate timed by the user.
Network-independent timing creates a problem for the existing formats because lack of synchrony between the user's and network clocks results in the user supplying data and control information to the network at a rate that, from the network's perspective, fluctuates about the nominal rate.
Existing formats typically either are incapable of handling such fluctuations, or are inefficient at doing so, i.e., they waste bandwidth in the process. This is especially true of formats designed for carrying subtrate-multiplexed communications. Subrate multiplexing is well known in the art. Typically, it involves the combining through synchronous time-division techniques of a plurality of independent communication signal streams intended for transmission over a common communication channel and each one of which has a bandwidth, i.e., a transmission rate, that is lower than, i.e., that is a subrate of, the bandwidth of the common channel, into a single stream having the common channel's bandwidth. For example, it is common to combine the signal streams of up to 20 subchannels, each having a bandwidth of 2.4 kilobits per second (Kbps) plus associated control signals (e.g., framing information), into a single ISDN B or DS0 channel signal stream having a total bandwidth of 64 Kbps by using a different one of each 20 successive time slots of the ISDN B or DS0 channel to carry the data signals and associated control signals of a different one of the 20 2.4 Kbps subchannels. Alternatively, fewer higher-rate subchannels may be carried by the single channel, by assigning a plurality of the time slots to a single subchannel. In subrate multiplexing, a plurality of channels are connected to a common--the multiplexed--channel during adjacent successive time intervals, or time slots, in a predetermined order that forms a repeating pattern of fixed duration, called a frame. Because the time slots are adjacent, i.e., not separated by idle intervals, transmissions of signals from individual ones of the plurality of channels are delimited by the beginning and end of the individual time slots and hence need no other delimiters. Also, any changes in the relative order of transmission on the common channel, such as substitution of one or more of the plurality of channels for another one or more of the plurality of channels in using one or more of the time slots of a frame, must be made known by the transmitter (the multiplexer), and to the receiver (the demultiplexer) prior to the change being effected. Consequently, the one of the plurality of channels that is the source of the signals transmitted during any time slot is identifiable from the position of those signals within the frame, and hence the signals typically do not include explicit addressing information. These characteristics make subrate multiplexing fairly inflexible, relative to other techniques such as statistical multiplexing, in terms of ability to quickly or efficiently respond to changes in the transmission rates of individual channels and in the transmission rate of the total information stream that the common channel carries. And because multiplexing combines a plurality of subchannels into one channel, the fluctuations in each subchannel multiply the worst-case effects on the multiplexed channel vis-a-vis an unmultiplexed one.
Many formats are in use for carrying subrate-multiplexed communications. Most are proprietary. The most widely-used public format is DDS SDM DS0-B, illustratively described by T. H. Murray in "The Evolution of DDS Networks: Part I", Telecommunications, Feb. 1989, pp. 39-47. The DS0-B format uses an 8-bit byte consisting of six data bits delineated by a control bit at one end and by a framing bit at the other end. A byte occupies a single time slot in a multiplexed DS0 channel. The DS0-B format accommodates multiple subrate signals (subchannels) in a single DS0 channel and distinguishes between them by means of framing patterns created by the framing bits. The information for a single subchannel appears in every fifth, tenth, or twentieth time slot of each frame, depending on the subrate. A secondary channel, providing an independent, low-speed channel that runs in parallel within the DS0's primary channel, is created by "usurping" for this purpose the control bit of every third, or less-frequent, time slot of a frame.